Does restorer need a scanner? Optical methods in canvas painting diagnostic

Prof. Joanna Szpor PhD, Katarzyna Górecka MA, Aleksandra Rzeszutek, Conservation and Restoration Faculty of the Academy of Fine Arts in Warsaw.

Does restorer need a scanner? Optical methods in canvas painting diagnostic
The principal rule of the conservation process is an individual approach to every piece of art. Easel paintings are varying regarding their forms (shape), technique and technology
(texture, stratigraphy, binder and pigments) and behave differently
in various conditions. In order to carry out initial researches of the particular object several noninvasive analyzes are performed. The documentation of the painting state before conservation includes photography in light diffused, reflected (or side), macro - photography and microscopic images of a particular section of the painting. Owing to the infrared photography (IR) the original artist's draft of the painting could be detected, X-Ray technology enables to notice previous composition, UV induced fluorescence photos relieve the varnish layer and show the range of over-painting. However, none of these methods give information about geometric shape of the object and metric analysis of its surface.
This gap in the field of initial optical analysis, may be filled with 3D scanning. There are several remote optical methods of 3D measurements: laser scanning, structured light projection and method of images correlation. In case of the unusual shape object like curvilinear canvas painting detailed analysis of its geometry is essential. A restorer has to face and answer several question e.g.: What is the current shape of the object? How did it look originally? How was its surface changed throughout the time? What was the reason of the deformation? A good example of such a restoration work is the conservation project of the curvilinear large-size canvas painting Adoration of the Magi from St. Aubin’s Cathedral church in Namur, supported by the National Science Centre under Grant DEC-2011/03/N/HS2/01936 Innovative measurement of deformations in large-size canvas paintings by remote optical methods and its application in documenting, designing and evaluation of technical conservation solutions. The painting shape was measured with a laser scanner 3D Surphaser 25HSX in the initial phase of work. Over 4500000 points with angular resolution of 0.0157rad and accuracy of 0.3 mm in the radial direction were acquired. Results of the scanning allowed finding profiles of canvas surface and assessing dimensions and curvature of the stretcher frame. It appeared that the painting shape was neither cylindrical nor ellipsoidal, but a kind of a ruled surface which could be described by a polynomial function. The measurements were necessary for the design and manufacturing of curvilinear work tables used for further conservation works. The shape of the canvas painting was measured second time after conservation treatment and than two scanning results were compared.
3D laser scanning method could be also used for analyzing ruggedness of the painting surface. Such an example is optical measurement of the two XVIII-th century portraits from the Tyszkiewicz familly collection, preserved in the National Museum in Warsaw. The group of the paintings has been subjected to the specific deformation of the surface in the relief form. The measurement of the paintings face deformation was carried out by using a prototype model of the laser scanner, equipped with a laser head (prod.Keynece) having accuracy of 7µm. The scanner was constructed under Grant DEC-2011/03/N/HS2/01936 supported by the National Science Center. The conservation works on the Tyszkiewicz familly portraits are performed as diploma works supervised by prof. Joanna Szpor from the Conservation and Restoration Faculty of the Academy of Fine Arts in Warsaw.
From conservators' point of view , we may say that a laser scanner slowly becomes an indispensable and daily work tool in the diagnosis and conservation methodology of the canvas paintings.